The extracellular connective tissue skeleton of adult mammalian hearts has at least 3 levels of organization: (1) a weave collagen network which surrounds groups of myocytes; (2) collagen struts that insert just lateral to the band interconnecting contiguous myocytes; and (3) collagen struts that connect myocytes and capillaries in such a manner as to aid in maintaining capillary patency during systole. Using scanning electron microscopy, transmission electron microscopy and polarizing light microscopy, we will investigate the origin, development, and disposition of the extracellular connective tissue network in rats and hamsters. The connective tissue skeleton is not present at birth in rats and hamsters and develops during the neonatal period (0-20 days). At this time, this system forms and develops presumably in response to the stresses developed in the heart, particularly in the left ventricle. The development of this system can be blocked with beta aminopropionitrile, an inhibitor of lysyl oxidase. Blockage results in the formation of ventricular ruptures and aneurysms.